This proposal sets the stage for clinical trials of gene therapy for hemophilia A, while simultaneously initiating studies on a novel vector system. We initially will determine the most effective and safest gene correction strategy for factor VIII (FVIII) deficiency in our mouse model. We have previously shown that functional VIII can be delivered from the suprabasal epidermis to the circulation in a transgenic mouse and a skin explant. We will improve FVII delivery by use of 1) a FVII resistant to inactivation and 2) a species-specific FVIII. We then use the optimal FVIII construct to deliver the gene in a retroviral vector to human keratinocytes. These cells which contain a high fraction of stem cells will be grafted to FVIII-deficient immunodeficient mice to show correction of FVIII-deficiency from transfected keratinocytes. In addition, we follow- up observations suggesting that intradermal injection of an FVIII transgene can result in long-term circulating FVIII. In a second section of the proposal, we seek to improve upon modest long-term correction achieved by mouse FVIII cDNA delivered in an adenoviral vector by tail vein injection. We test a number of variables, including improved gene construct and short-term immunosuppression. We also use a "gutted" adenoviral vector as a delivery vehicle to improve results. These studies should provide sufficient information concerning optimal modes of therapy to justify attempts to correct a dog model of hemophilia A in the later years of the grant. In a fourth Specific Aim, we initiate studies on a novel vector, the endogenous L1 retro-transposon and determine the efficacy of this promising vector in the delivery of the factor IX gene in cultured cells.